John Green is used to seeing passion from his worldwide admirers. But the best-selling author of The Fault in Our Stars admits he was floored last month during a Dallas fan visit.

As he drove around the event promoting the upcoming film version, Green was able to see the entire spirited crowd of about 4,000 people.

"It was pretty mind-blowing," says Green. "You can think about 4,000 people in the abstract. But when you actually see them there, that means a lot."

It's more than just personal gratification. The filmmakers and 20th Century Fox are counting on the self-professed "nerd" author to attract his ever-growing army of devotees to the multiplexes when the love story between two young cancer patients opens in theaters on Friday.

Even with proven box office stars like Shailene Woodley and Ansel Elgort (both stars in the Divergent franchise), producer Wyck Godfrey says it was Green the fans sought out in Dallas — and who can provide serious pull for the film.

"John is a nerd rock star," says Godfrey, who produced the pandemonium-filled Twilight franchise. "With Twilight they were screaming bloody murder for the actors. In this case they are screaming louder for John than anybody."

"Like in Twilight, it was a little uncomfortable for John to see this for the first time," adds Godfrey. "It's like, 'My God, what have I created?' But he understands now."

Green says he never imagined the 2012 novel, his fourth, would vie for box office supremacy.

"The female romantic lead has tubes in her nose in every scene and the male romantic lead is an amputee," says Green. "This is not a movie where you go to make money, this is not X-Men 5 or whatever."

Nonetheless, Paul Dergarabedian, analyst for the movie tracking firm Rentrak, says Stars success is a "foregone conclusion" with Green playing a "key role in the film's marketing" along with an endearing performance by Woodley.

Much of this heat is the result of the best-selling novel, which has sold 10.7 million copies worldwide. But Green has further stoked the fires as a social media presence from the Stars set.

He has been tweeting regularly to more than 2.2 million followers and posting set pictures on Instagram — Elgort helped set up the account. Green is already a longtime YouTube star with his brother Hank on a variety of channels (including Vlogbrothers) totaling more than 8.6 million subscribers.

The social media message of his excitement about the movie has assured fans that the screen version is a faithful rendition of the uncompromising love story.

"I was on set most every day of filming," says Green. "My voice was always heard. I never felt excluded. That is not the usual story for Hollywood."

A new generation of adults, primarily mothers, have been the latest Fault converts — "that's where the story has really grown," says Green. "It's not something I expected." His big tent continues to grow in time for the film, while his message of acceptance resonates "in a society which doesn't encourage community," says Woodley.

"He lets people know it's OK to be a weirdo and a freak," she says. "There is a real reverence for him that I have never seen before, which is why his popularity is just growing and growing."